Project Summary/Abstract: Aortic valve disease is an increasingly prevalent cause of morbidity and mortality, with no current effective pharmacological treatment. An underlying pathology is often aberrant differentiation of valvular interstitial cells (VICs) leading to altered composition of extracellular matrix (ECM) and calcification. Although various factors involved in VIC differentiation have been described, the chromatin modifiers required to maintain mesenchymal identity of VICs remain largely unknown. Histone deacetylases (Hdacs) lack intrinsic DNA-binding domains but modify chromatin via their interactions with transcription factors, co-factors, and large multiprotein transcriptional complexes. We recently published that mouse embryos lacking Hdac3 within the second heart field cardiac progenitor cells exhibit complete embryonic lethality and severe cardiac developmental defects, including bicuspid and hyperplastic aortic valve. Our preliminary data suggest a novel and unexpected role of Hdac3 in postnatal aortic valve homeostasis. Hdac3-null aortic valves exhibit upregulation of a discrete set of chondrogenic genes, which are frequently elevated in human diseased aortic valves. In murine aortic valves, Hdac3 recruits components of the Polycomb Repressive Complex 2 (PRC2), including methyltransferase Ezh2, Eed, and Suz12 to enrich trimethylation of lys27 on histone H3 (H3K27me3), a gene silencing mark, at the regulatory chondrogenic gene loci. The goal of this research program is to identify how Hdac3 regulates mesenchymal identity of aortic valvular interstitial cells in both humans and mice. In addition, proposed studies will identify the mechanisms by which different kinases involved in signaling pathways regulate the phosphorylation, function, and chromatin recruitment of Hdac3. Despite intense study in the area of epigenetics, very little is known about the role of epigenetic and chromatin modifiers in the field of aortic valve biology. The set of experiments outlined in this proposal have broad significance not only for understanding how signaling pathways intersect with chromatin modifiers to regulate homeostasis of aortic valves, but also could be highly applicable to the entire field of cardiovascular diseases.